Remove sys/sysctl.h on linux platform

This include is useless and can be removed.
This syscall has been strongly deprecated for ages and the plugin doesn't use it any more.  On old architectures, this include is benign as long as it's unused, on new ones (such as x32) it results in a build failure.
Fix bug #11477

panel-plugin/os.h

@@ -170,7 +170,6 @@
 #elif __linux__             /* L I N U X */
 #  include <stdio.h>
 #  include <sys/param.h>
-#  include <sys/sysctl.h>
 #  include <stdlib.h>
 #  include <stdarg.h>
 #  include <unistd.h>